I have my sub facing the rear of the cab. I have an 03 4dr supercab. The window rattles, the rear tool cover/cup holder rattles and the seats rattle but they're coming out anyway. I'm adding a second sub, going away from the rear facing ported box to 2 separate down firing sealed boxes and adding a second box. People can sit on them instead of the jump seats. Haha.
I only have about 3" of space between the sub and the jump seats. My stereo guy (the guy who sells me stuff) said I only have to worry about having enough room for the excursion.